Reaction products of melamine or related species with organic phosphoric acids are known. For example, U.S. Pat. Nos. 4,154,930; 4,201,705; 4,338,245; 4,338,246; 4,342,682 and 4,480,093 are directed to certain amine salts of organic phosphoric acids. The amine salts are useful as flame retardant additives in certain polymeric compositions. In addition to the above mentioned patents, melamine has been reacted with phosphorous pentoxide, phosphoric acid or other phosphorous-containing compounds to produce reaction products that also are useful as flame retardants for synthetic resins as disclosed in U.S. Pat. Nos. 2,544,706; 4,010,137 and 4,080,501. The preparation of melamine flame retardants by techniques disclosed in the above identified patents tends to be hazardous or unattractive. For instance, the reaction of phosphorous oxychloride with pentarythritol, followed by reaction with melamine, is unattractive because it involves generation of six moles of hydrochloric acid per mole of product. In addition, the technique involves the handling of toxic phosphorous oxychloride and highly corrosion-resistant apparatus must be employed. In alternative routes, pentarythritol and melamine are reacted with fuming phosphoric acid, but this technique is equally unattractive. Further improvements are needed in preparation of flame retardants, especially of the melaminephosphorous containing type without the hazards and disadvantages associated with current techniques.